Chapter 6 The Muscular System 175 Copyright Goodheart-Willcox Co., Inc. individuals. As these findings suggest, exercise training over time can result in changes in fiber types within an individual. Today it is accepted that fast-twitch fibers can be converted to slow-twitch fibers with endurance training and that within the fast-twitch fibers conversions from Type IIb to Type IIa fibers can occur with heavy resistance (strength) training, endurance training, and concentric and eccentric isokinetic training. Fiber Architecture Another factor that affects functions of skeletal muscles is fiber architecture. Fiber architecture refers to the ways in which fibers are arranged within the muscle. The two major categories of muscle fiber arrangement are parallel and pennate. Parallel Fiber Architecture In parallel fiber architecture, the fibers run largely parallel to each other along the length of the muscle. As Figure 6.9A shows, this arrangement may result in muscle shapes that are fusiform (wide in the middle and tapering on both ends), bundled, or triangular. Examples of muscles with this type of architecture are the biceps brachii (fusiform), rectus abdominis (bundled), and pectoralis major (triangular). The individual fibers in the parallel architecture typically do not run the entire length of the muscle. Instead, the individual parallel fibers are connected to neighboring fibers, which promotes contraction when the muscle is stimulated. This fiber arrangement enables shortening of the muscle and the movement of body segments through large ranges of motion. of influence are the characteristics of their skeletal muscle fibers. Slow-Twitch and Fast-Twitch Fibers Skeletal muscle fibers can be divided into two umbrella categories: slow-twitch (Type I) and fast-twitch (Type II). As the names suggest, the fast-twitch fibers contract much faster than slow-twitch fibers. Because sufficient variation exists among the fast-twitch fibers, they too have been divided into two categories: Type IIa and Type IIb. The contraction speed of Type IIa fibers falls somewhere between the slow-twitch fibers and Type IIb fibers, which are the classic fast-twitch fibers. Type IIb fibers contract very rapidly, in about one-seventh the time required for slow-twitch fibers to contract. As a result, Type IIb fibers also fatigue rapidly. Although all of the muscle fibers in a motor unit are of the same type, most skeletal muscles include motor units of both fast-twitch and slow-twitch fibers. The fast-twitch/slow-twitch ratio varies from muscle to muscle and from person to person. How can muscle fiber type distribution affect physical performance? A large percentage of fast-twitch fibers can contribute to a performer’s success in events requiring fast, powerful muscular contraction, such as sprinting and jumping. Endurance events, such as distance running, cycling, and swimming, require effective functioning of the more fatigue-resistant slow-twitch fibers. However, the fiber type distributions of both elite strength-trained and elite endurance- trained athletes have been shown to fall within the range of fiber type compositions found in untrained 0 S = Stimulus S S S S S S S S S S S S S S S S S S S S S Tension Time (msec) A B C Goodheart-Willcox Publisher Figure 6.8 Tension developed in a muscle in response to a single stimulus (A), in response to repetitive stimulation (B), and in response to high-frequency stimulation, or tetanus (C).